1. Field of the Invention
The present invention relates to biotechnology in the fields of agriculture and energy production. More specifically, the present invention relates to the production of hydrogen from renewable polysaccharides through enzymatic catalysis of polysaccharides substantially into hydrogen and carbon dioxide.
2. Discussion of Related Art
Climate change and the eventual depletion of the world's fossil-fuel reserves are threatening sustainable development (Morris, 2006; Hoffert et al., 2002; Farrell et al., 2006). Hydrogen is a mobile energy carrier and is abundant, clean, and of course does not contain carbon. According to the United States Department of Energy (2004), R&D priorities for the future hydrogen economy include (i) decreasing hydrogen production costs, (ii) finding viable methods for high-density hydrogen storage, (iii) establishing a safe and effective infrastructure for seamless delivery of hydrogen from production to storage to use, and (iv) dramatically lowering the costs of fuel cells and improving their durability.
Hydrogen is a very promising alternative for storing energy rather than employing fossil fuels. Hydrogen can be utilized in a fuel cell, which is an electrochemical device that converts the chemical energy of a reaction directly into electrical energy. A fuel cell is a highly efficient device for generating power and heat. Fuel cells offer the potential to significantly decrease reliance on fossil fuels. However, fuel supply is one of the major obstacles preventing widespread commercialization of such devices.
Most fuel cells operate by using hydrogen gas as the reactant, generally made by reforming (converting) a hydrogen compound. Hydrogen-rich fossil fuels are most commonly reformed using catalytic steam reforming, auto-thermal reforming, or catalytic partial-oxidation reforming. For fuel-cell technology to sustain our energy needs, though, renewable sources of hydrogen are required. Examples of non-fossil-fuel based hydrogen generation include electrolysis of water using solar or wind power, hydropower, or geothermal energy; thermochemical water splitting; waste gases at industrial sites; gasification of biomass; and bio- or photobiological systems that produce hydrogen gas upon digesting organic components or upon absorption of sunlight.
Unlike electricity, which must be used as it is produced, hydrogen can be stored until needed. The low density of hydrogen, however, translates into a low energy density, particularly in comparison with traditional fuels. Even factoring into account the higher efficiency of a fuel cell compared with traditional power-generation methods, the low energy density of hydrogen dictates potentially inhibitive storage and transportation methods.
Polysaccharides can be a good means of storage for hydrogen because polysaccharides contain about 6.2% by mass H2 per sugar unit, and when it reacts with water, it can produce 12 moles of dihydrogen. So the potential hydrogen storage capacity is around 15%. A material with 15% hydrogen storage capacity exceeds even long-term objectives for hydrogen-storage technologies, according to U.S. Department of Energy goals (Schlapbach and Zuttel, 2001). In order to actually extract hydrogen from polysaccharides and water, the overall reaction C6H10O5+7 H2O→6 CO2+12 H2 needs to take place, where C6H10O5 represents glucan repeat units contained in biomass, such as starch or cellulose.
There have been several paradigms for converting biomass to hydrogen: (1) direct polysaccharide gasification (Antal et al., 2000); (2) direct glucose chemical catalysis (Cortright et al., 2002; Huber et al., 2003); (3) anaerobic fermentation (Das and Veziroglu, 2001; Hallenbeck and Benemann, 2002); and (4) ethanol fermentation (Lynd et al., 2002; Zhang et al., 2006; Zhang and Lynd, 2004) followed by ethanol reforming (Deluga et al., 2004). The conventional chemical methods have low hydrogen yields (<60%) and require high reaction temperatures (e.g., 500-900 K) (Antal et al., 2000; Cortright et al., 2002; Huber et al., 2003). Anaerobic hydrogen fermentation is well known for low efficiency with a maximum yield of 33% (Das and Veziroglu, 2001; Hallenbeck and Benemann, 2002). The combination of ethanol fermentation and reformation can produce 10 H2 per glucose unit (83% yield). Considering 5-10% fermentation loss and around 5% reforming loss (Deluga et al., 2004), the practical hydrogen yield through ethanol could be approximately 75% of the maximum yield (the maximum yield being 12 H2 per glucose unit).
Storage and distribution of solid polysaccharides to be converted to hydrogen could address many challenges of the hydrogen art today. If a practical process and apparatus could be used directly on board mobile applications, such as vehicles, several problems associated with hydrogen-storage devices could be solved. Namely, energy loss for hydrogen compression or liquefaction would be avoided. Additionally, high temperatures for H2 desorption would no longer be necessary. Furthermore, solid hydrogen storage materials lifetime and hydrogen refilling time are not problems to practical applications any more. Also, storage and distribution of carbohydrate is very safe as compared to gaseous hydrogen.
The biochemical pathway for molecular hydrogen production from elemental hydrogen is known. An electron on photosystem I (either isolated photosystem I or photosystem I in thylakoids) is excited, typically by light, to a higher energy, resulting in the donation of an electron to an exogenous electron carrier that in turn can transfer electrons to the enzyme hydrogenase. In this process, oxidized photosystem I can then extract an electron from an electron donor, either directly or through an electron transfer chain, such as that found in the thylakoid membrane. Where water acts as the electron donor, the electron transfer chain includes photosystem II. Meanwhile, two reduced electron carrier molecules are able to donate electrons to the enzyme hydrogenase. Hydrogenase combines two electrons with two protons to form a hydrogen molecule.
Aerobic oxygen-producing photosynthetic organisms, or subcellular components from such organisms, have been used previously to make hydrogen gas (Benemann et al., 1973; Rosen and Krasna, 1980; Rao et al., 1978). The components of cell-free (i.e., in vitro) systems reported in these references require isolated thylakoids or solubilized photosystem I from thylakoids; an electron donor, such as water or an artificial electron donor such as dithiothreitol or ascorbic acid; a hydrogenase capable of accepting electrons from photosystem I that can catalyze the combination of two electrons and two protons to form molecular hydrogen when electrons are received from an electron donor that can be oxidized by the hydrogenase; and an exogenous electron carrier that is capable of accepting electrons from photosystem I and can donate electrons to the hydrogenase.
At least two groups of oxygen-producing photosynthetic organisms are capable of producing hydrogen in vivo. These include cyanobacteria and green algae. Cyanobacteria generally use the enzyme nitrogenase to produce molecular hydrogen. Electrons used in this molecular hydrogen-producing process are derived from stored carbohydrate and are used to reduce ferredoxin, which is the immediate electron donor for nitrogenase (Markov et al., 1995). Hydrogenase can also catalyze molecular hydrogen production in cyanobacteria. In cyanobacteria, molecular hydrogen production is inhibited by oxygen and/or light.
Green algae can also photoevolve (i.e., produce) molecular hydrogen via hydrogenase. The pathway of electron transfer is currently unknown. The source of electrons for the process has been shown to be endogenously fermented carbohydrate (Klein and Betz, 1978). Hydrogen production stops in the presence of carbon dioxide (Vatsala and Seshadri, 1985), indicating that the electron sink of carbon dioxide reduction is a better competitor for photosynthetic electron flow than hydrogenase.
Molecular hydrogen (H2) has a number of commercial uses. Molecular hydrogen is used for the production of ammonia; in petroleum refining, where H2 is used throughout a typical refinery; in the chemical-synthesis industries, when conversion of a double carbon-carbon bond to a single C—C bond is desired, or of a triple carbon-carbon bond to a single or double bond; in the food industry for hydrogenation of vegetable oils; and in electronic-circuitry manufacture. Hydrogen is also used extensively today to make methanol, fertilizers, glass, refined metals, vitamins, cosmetics, soaps, lubricants, and cleaners. Further, pure hydrogen is an excellent fuel, both in traditional combustion engines as well as in fuel cells, and produces only water vapor when oxidized with oxygen. Liquid hydrogen can also be used as a fuel, such as in space vehicles.
The state of the art for hydrogen fuel generation today has many challenges, such as limited yield, high energy input required, high costs, and additional purification steps to ensure that the fuel is sufficiently clean. What is needed is an inexpensive method of generating hydrogen in very large quantities, sufficient to support extensive use of fuel cells and other uses of hydrogen. Especially desirable are practical processes, compositions, kits, and apparatus to convert renewable feedstocks, such as abundant biomass containing polysaccharides, directly into hydrogen. It is desired that the process is capable of high yields to H2 and of good mass and energy efficiency, and ultimately of low cost.